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S.W. Nelson

Publications and source records attributed to S.W. Nelson.

16 recordsLinked to original sources
USGS1996

Evidence for multiple mechanisms of crustal contamination of magma from compositionally zoned plutons and associated ultramafic intrusions of the Alaska Range

Models of continental crustal magmagenesis commonly invoke the interaction of mafic mantle-derived magma and continental crust to explain geochemical and petrologic characteristics of crustal volcanic and plutonic rocks. This interaction and the specific mechanisms of crustal contamination associated with it are poorly understood. An excellent opportunity to study the progressive effects of crustal contamination is offered by the composite plutons of the Alaska Range, a series of nine early Tertiary, multiply intruded, compositionally zoned (Peridotite to granite) plutons. Large initial Sr and Nd isotopic contrasts between the crustal country rock and likely parental magmas allow evaluation of the mechanisms and extents of crustal contamination that accompanied the crystallization of these ultra-mafic through granitic rocks. Three contamination processes are distinguished in these plutons. The most obvious of these is assimilation of crustal country rock concurrent with magmatic fractional crystallization (AFC), as indicated by a general trend toward crustal-like isotopic signatures with increasing differentiation. Second, many ultramafic and mafic rocks have late-stage phenocryst reaction and orthocumulate textures that suggest interaction with felsic melt. These rocks also have variable and enriched isotopic compositions that suggest that this felsic melt was isotopically enriched and probably derived from crustal country rock. Partial melt from the flysch country rock may have reacted with and contaminated these partly crystalline magmas following the precipitation and accumulation of the cumulus phenocrysts but before complete solidification of the magma. This suggests that in magmatic mush (especially of ultramafic composition) crystallizing in continental crust, a second distinct process of crustal contamination may be super imposed on AFC or magma mixing involving the main magma body. Finally, nearly all rocks, including mafic and ultramafic rocks, have ( 87 Sr/ 86 Sr) i that are too high, and ε(T) Nd that are too low, to represent the expected isotopic composition of typical depleted mantle. However, gabbro xenoliths with typical depicted-mantle isotopic compositions are found in the plutons. This situation requires either an additional enriched mantle component to provide the parental magma for these plutons, or some mechanism of crustal contamination of the parent magma that did not cause significant crystallization and differentiation of the magma to more felsic compositions. Thermodynamic modeling indicates that assimilation of alkali-and water-rich partial melt of the metapelite country rock by fractionating, near-liquidus basaltic magma could cause significant contamination while suppressing significant crystallization and differentiation.

Journal of Petrology↗
USGS1984

A transect of metamorphic rocks along the Copper River, Cordova and Valdez Quadrangles, Alaska: A section in The United States Geological Survey in Alaska: Accomplishments during 1982

The lower Tertiary Orca Group is juxtaposed against the Upper Cretaceous Valdez Group along the Contact fault system (Winkler and Plafker, 1974, 198; Plafker and others, 1977)(fig. 33). In both groups, turbidites are the dominant rock type, with lesser mafic volcanic rocks (table 10). The Valdez Group, on the north, has traditionally been considered to be of higher metamorphic grade than the Orca Group (Moffit, 1954; Tysdal and Case, 1979; Winkler and Plafker, 198; Winkler and others, 1981). In 1982, we made a transect across the regional strike of the rocks and the contact between the two groups. The transect area follows the Copper River for 85 km from the Cordova quadrangle north into the Valdez quadrangle and extends for about 25 km on either side of the river (fig. 33). We planned, by systematic sampling of the area, to examine the metamorphic differences between the Orca and Valdez Groups. We found, however, that a strong thermal metamorphic event has overprinted and obscured regional metamorphic relations. We believe intrusion of Tertiary granite (fig. 33) to be responsible for this metamorphism. (Figures 33 and 34 and tables follow this article.)

Alaska↗